Apparatus for the injection molding of plastic objects

ABSTRACT

In the formation of plastic objects having an imporous dense film-like surface enclosing a foam-like porous core, the plastic material and an expanding agent are mixed, heated to a hot plastic compound, and transported through an enclosed axially elongated passageway by a threaded screw member. The threaded screw member is shaped to provide a flow path of variable transverse cross sectional area along its length. By axially displacing the screw member, dosed amounts of hot plastic compound, collected at the outlet end of the passageway, can be extruded for injection into a mold.

United States Patent 1 1 3,728,053 Stillhard et al. [4 Apr. 17, 1973[54] APPARATUS FOR THE INJECTION 3,263,276 8/1966 Maier ..l8/l2 SA xMOLDING OF PLASTIC OBJECTS 3,391,051 7/1968 Ehrenfreund et al ..l8/l2 FUX [76] Inventors: Bruno J. Stillhard, Wildeggstr. 38, FOREIGN PATENTSOR APPLICATIONS I 9000 Saint Gallen; Fredy Z. Scheifele m G un 92 4Niederuz 230,910 4/1960 Australia ..l8/3O SQ W11, both of swl z PrimaryExaminer-H. A. Kilby, Jr. [22] Filed: Aug. 26, 1970 Att0rney-MOGlew &Toren [21] Appl. No.: 67,020 [57] ABSTRACT In the formation of plasticobjects having an imporous [3O] Forelgn Apphcatlon Pnonty Data densefilm-like surface enclosing a foam-like porous Sept. 4, I969 Switzerland..i 3407/69 9" the Plastic material an expanding agent ale mlxed, heatedto a hot plastic compound, and trans- 52 vs. C]. ..425/4, 425/208,425/245, hmugh encmsed axany elmgaed 259/191 passageway by a threadedscrew member. The threaded screw member is shaped to provide a flow[251;] ..B29d 27/04 pathvof variable transverse cross Sectional areaalong 1 It 0 "18/5 128A its length. By axially displacing the screwmember, 18/12 30 30 30 SR; 425/4 dosed amounts of hot plastic compound,collected at 245 the outlet end of the passageway, can be extruded forinjection into a mold. [56] References Cited 14 Claims, 2 DrawingFigures UNITED STATES PATENTS 3,226,766 l/l966 Zelewski et al 18/12 SEUX 4 H 70 2 4? i 31' 16 d Mb I I 1 421m 1 I a I i" l8 7 26 l 25a Zr 2325 41 42a l 32 P 9 I l BRUNO STILL AR BY FREDY SCHEIF E- Wc 'aramATTORNEYS SUMMARY OF THE INVENTION The present invention is directed toapparatus for the 5 injection molding of plastic objects and, moreparticularly, it is directed to an arrangement for mixing, heating, andtransporting the materials forming a plastic compound within anelongated passageway by means of a threaded screw member. The plasticobjects injec- 1 tion molded from the materials processed through thismethod and apparatus have an imporous dense film skin enclosing afoam-like porous core.

In the past, it has been suggested to melt the materials for theproduction of such plastic objects, normally referred to as foamedobjects, and to inject the melted materials along with a foaming agentin gaseous form into a mold. This injection molding process provedcomplicated in practice and was not completely satisfactory, because itwas not possible to obtain a substantially homogeneous distribution ofthe foaming agent in the melted plastic materials.

Based on this disadvantage, the plastic material and the foaming agentwere mixed in dry form and the mixture was subsequently melted andcharged into a mold. In the production of foamed objects, mold cavitiesare frequently filled with a foam plastic. Accordingly, a method wasselected where the melted materials were prepared in an extruder andsupplied into a vessel containing a piston and, subsequently, the meltedmaterial was extruded from the vessel into the mold by displacement ofthe piston.

In contrast to conventional injection molding of homogeneous denseobjects formed of thermoplastic materials, the injection forces arepartly much lower depending on the desired form of the skin or surfaceof the object.

From these findings, it was concluded that the conventional injectionmolding process could be used for the production of these structurallynew objects by utilizing different parameters. However, considerabledifficulties were encountered, contrary to expectation, since either theprocessing cycles were uneconomical, due to the poor relationshipbetween the plasticizing unit and the size of the object being formed,or because no satisfactory relationship could be established between theplasticizing energy and the injection energy. Therefore, the primaryobject of the present invention is to overcome the difficultiesexperienced in the prior art by melting the plastic material and theextending agent necessary for the production of the foam-like porouscore structure and by supplying dosed amounts of the mixed material foruse in injection molding. The mixing and heating of the materials isaccomplished by a threaded screw member which is both rotatable andaxially displaceable within an elongated passageway from which thehot-plastic compound is extruded.

In accordance with the present invention the materials required for theplastic compound are melted at the inlet end of the passagewaycontaining the screw member.

In another embodiment of the invention, the materials can be melted bysimultaneously rotating and retracting the screw member while the hotplastic compound accumulates adjacent the outlet end of the passageway.

Further, the apparatus in accordance with the present invention providesa hot plastic compound for an injection molding operation in whichplastic objects are formed having an inporous dense film surface or skinwhich encloses a substantially uniform foam-like porous core. Theplastic material and the extending agent are changed as a mixture orseparately into an elongated passageway containing a threaded screw 0member which extends from the inlet end of the passageway to a positionspaced closely from its outlet end from which the hot plastic compoundis delivered into a mold. The threaded screw member is rotatable andaxially displaceable within the passageway and the groove portions ofthe screw member between its threads have a variable transverse crosssectional area along the length of the screw member with the transversecross sectional area decreasing in the direction of flow of the materialalong the screw member. The passageway is formed by a first wall sectionhaving a cylindrical shape and a second wall section forming thecollecting chamber and outlet end of the passageway. The second wallsection contains, in series, a diverging transition section connected tothe first wall section, a cylindrical section and a conical section withthe outlet for the hot plastic compound located at the apex of theconical section. The screw member is threaded for its full extent withinthe first wall section and for the portion ofits length located withinthe transition section of the second wall section. The threaded lengthof the screw member has an outside diameter which correspondssubstantially to the inside diameter of the first and second wallsections within which it is positioned. At its end adjacent the outletend of the passageway, the screw member has a screw head extendingaxially from its threaded portion with a cylindrically shaped partextending from the threaded portion and a conically shaped part formingthe end of the screw member within the outlet end of the passageway. Thecylindrically shaped part of the screw head is located within thecylindrically shaped portion of the second wall section and has an axiallength which is shorter than the axial length of the cylindricallyshaped portion by a dimension equal to the maximum axial displacementpath of the screw member within the passageway.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this specification. For a better understanding of the invention,its operating advantages and specific objects attained by its use,reference should be had to the accompanying drawings and descriptivematter in which there is illustrated and described preferred embodimentsof the invention.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a longitudinally extendingview, partly in section, of an apparatus, in accordance with the of thepresent invention, located between the lines A-B in FIG. 1. A

DETAILED DESCRIPTION OF THE INVENTION In F [6.1, an apparatus is shownfor mixing and melting plastic materials for subsequent use in theinjection molding of plastic objects which have an imporous dense filmsurface or skin enclosing a substantially uniform foam-like porous core.In forming the plastic compound for use in the injection moldingoperation, a mixture of plastic material and an expanding agent areintroduced into a screw plasticizer unit 11 having an inlet end 11a andan outlet end 11b. Adjacent the inlet end 1 la of the plasticizer unit 11 is a mixer l positioned below a hopper 2 with a slide valve 3 locatedwithin the lower portion of the mixer and associated with a slide valveactuator 4 for regulating the discharge from the mixer. Below the slidevalve 3, an outlet 5 is connected to a line 6 which empties into a feedhopper at the inlet end of the plasticizer unit 1 1. It is possible tofeed the material from the mixer into the unit 1 1 either continuouslyor in selected charges.

To form the plastic objects with a dense surface film and a porous core,a divisible mold M is formed by mold halves 7 and 8 which, incombination, form a mold cavity 9 for the plastic objects. To move themold halves 7 and 8 and thus open and close the mold M dif ferent moldclosing devices can be utilized, that is hydraulic, hydraulic-mechanicalor strictly mechanical types with suitable closing and locking elementsfor resisting the pressing forces. Within the outlet end 111; of theplasticizer unit 11, a collecting chamber 20 is provided from which thehot plastic compound is provided through a controllable shut-off device70 into the mold cavity 9.

The plasticizer unit comprises a cylindrically shaped first wall sectionwhich extends from the inlet end 11a to a second 'wall section composedof a frusto-conically shaped transition section 16, a cylindricallyshaped collecting section 17 and a conically shaped head portion 18which forms the outlet end of the unit with a nozzle 19 located at theapex of the head 18. Adjacent the inlet end 1 la, an inlet opening 12 isprovided through the first wall section 15 through which the materialsare introduced into the unit 11. Due to the configuration of the varioussections which make up the plasticizer unit 1l,it has a plurality ofdifferent transverse cross sectional areas between its inlet opening 12and its nozzle or outlet opening 19. At the outlet end 11!: of theunit,a collection chamber 20 is located between the frusto-conicaltransition section 16 and the nozzle l9,and this chamber has a largercross sectional area than exists in the passageway adjacent the inletopening 12.

Extending axially through the first wall section and into the secondwall section of the unit I 1 is a plasticizing screw member arrangedboth to rotate and to be axially displaced through the passageway. Forits full extent within the first wall section 15 and within thetransition section 16 of the second wall section, the screw member 25comprises a core 250 and threads 23 extending outwardly from the coreinto sliding contact with the interior surface of the passageway so thatgrooves 24 are formed between the threads and outwardly from the surfaceof the core. As can be noted in FIG. 1, between the inlet opening 12into the transition section 16 of the unit ll,the open flow pathprovided by the grooves 24 has a variable transverse cross sectionalarea with the area decreasing in the direction of flow. In addition tovarying the diameter of the core 25a,the variable cross sectionalareascan be provided by varying the pitch of the threads 23. The screwmember 25 has an outlet end 26 of increased cross sectional size so thatit corresponds to the configuration of the transition section 16 of thesecond wall section. As indicated in FIG. 1 the screw member is threadedfor the extent of the transition section. It has been found advantageousto increase the number of threads on the screw 25 in the range of thetransition section 16. Extending axially from the outlet end 26 of thethreaded portion of the screw member is a head 27 which is comprised ofa cylindrically shaped part 27a and a conically shaped part 2711extending from the cylindrically shaped part toward the outlet nozzle19. The conically shaped part 27b of the head 27 has a configurationwhich corresponds substantially to the configuration of the innersurface of the conically shaped section 18 of the second wall section ofthe unit 11. Within the range of the cylindrically shaped part 270 ofthe head of the screw member a return flow block 28 is arrangedextending inwardly from the inner surface of the cylindrically shapedsection 17 into an annular shaped flow channel 30 formed in thecylindrically shaped part 27a. Under certain operating conditions theflow channel 30 connecting the end of the threaded section of the screwmember with the collecting chamber 20 admits flow of the plasticcompound into the collecting chamber. However the disposition of thereturn flow block within the flow channel interrupts any flow passagewhen the flow block bear s tightly against the surface of the screwmember 25 adjacent its outlet end 26.

On the exterior of the cylindrically shaped section 17 of the secondwall section a heating sleeve 31' is positioned and,similarly,on theexterior surface of the first wall section 15 a pair of axially spacedheating sleeves 31", 31" are located. These heating sleeves areconnected through lines 32 with a suitable energy source. Heating and/orcooling systems for use in injecting molding machines for thermoplasticand thermosetting materials are well known.

To rotate the screw member 25 within the passageway formed by the firstand second wall sections, a hydraulic motor 40 is used which isconnected to the screw member by a transmission 41. For the axialdisplacement of the screw member along with its outlet end 26 and head27,a displacement cylinder 42 is utilized.

To operate the apparatus for the entire injection molding procedure, acontrol unit 50 is used which comprises all the control and regulatingdevices needed both for themovement and the thermal behavior of theapparatus. As shown in FIG. 1, this control unit is connected to anelectric potential source 51 and to a pressure medium source 52 which,in turn, is associated with a tank 53 containing a pressure medium 54.

The following methods for the injection molding of plastic objectshaving a different surface and core structure can be carried out usingthe apparatus described above. The thermoplastic and thermosettingmaterials used in injection molding normally available in the form ofgranules and are mixed with the expanding agent in the plasticizing unitto a hot-plastic compound with either the plastic material and theexpanding agent mixed as solids in a homogeneous blend,or with theplastic material having a uniformly distributed film of liquid expandingagent on its surface, or the expanding agent is mixed homogeneously withthe plastic material before granulation with the result that theexpanding agent is contained in the plastic granules to be processed.Any such mixture of plastic material and expanding agent is chargedthrough the feed hopper l0 and the inlet opening 12 into theplasticizing unit 11. In this unit, the mixture is melted by the energyof the rotation imparted to the screw member 25 and by additionalheaters, if necessary, which can be formed as sleeve elements 31 31 and31 located on the cylindrically shaped section 17 moving of the secondwall section and on the first wall section 15. The melting process canbe effected selectively in one of the following types of operations:

a. In the position of the screw member axially displaced toward theinlet end lla and where the threads of the outlet end 26 of the screwmember bear in sliding engagement against the inner surface of thetransition section 16 of the passageway, the collecting chamber 20 isfilled gradually with dosed amounts of the foamable plastic as thethreaded screw member 25 rotates.

b. As the plastic compound is delivered into the collecting chamber 20by the rotating threaded screw member 25 the accumulated material in thecollecting chamber will effect a rearward displacement of the screwmember if it is axially displaced toward the outlet end 1 lb of thepassageway.

c. A control retracting movement of the screw member 25 can be carriedout regardless of the progress of the filling of the collecting chamber20, in order to secure, primarily before the end of the charging of adosed amount into the collecting chamber, a final delivery of theplastic material from the range of the transition 16 into the range ofthe head end 27 of the screw member which is located within thecollecting chamber 20.

As can be seen from the illustration in FIG. 1, the mixer 1 used insupplying the mixture of plastic material and expanding agent to thehopper can be separated from the injection molding apparatus. It is notnecessary to provide a delivery pipe from the mixer into the feed hopperof the molding apparatus, rather the charging of the mixture into theapparatus can be effected from a container.

After the specific amount or dosed quantity of the plastic material isdelivered into the collecting chamber, which takes place with theshut-off element 70 in the closed position, by supplying hydraulicpressure to the displacement cylinder 42, a piston 42a (shown in dottedlines) which is connected to the screw member 25 axially displaces thescrew member and, with the element 70 open,the hot plastic compound isextruded from the collecting chamber 20 into the mold cavity 9. Duringthis portion of the operation, the annular shaped return flow block 28bears against the surface of the outlet end 26 of the screw member andprevents any backflow of the hot plastic material through the flowchannel 30 into the grooves 24 which form the flow path over the screwmember. After the injection of the hot plastic compound into the mold, anew dosed quantity of the compound is advanced into the collectingchamber for the next injection operation.

The specific control measures in injection molding apparatus, such asthe control of the heating or cooling in dependence on the temperatureand/or on the torque absorbed by the screw member, the dynamic pressureon the piston 42a connected to the screw member in the displacementchamber 42, the speed of the screw member provided from the hydraulicdriving motor 40 over the transmission 41 and the injection velocity areall effected in a known manner by the control unit 50.

The advantage of this arrangement is that the conventional plasticizerunit merely needs to be connected with a special second wall sectionhaving the transition section 16, the cylindrical section 17 and theconically shaped section 18 and with the head 27 and the outlet end 26of the screw member shaped to correspond to the surfaces of the secondwall section to carry out the present invention. With the driving poweravailable in normal injection molding apparatus it is readily possibleto plasticize the necessary amount of the foaming plastic material inthe collecting chamber, which is generally larger compared to the normalinjection molding space, without overloading it, and then to inject thematerial from the collecting chamber into the mold cavity using lowerinjecting forces for these molding methods. However, since lowerinjection molding forces are required, particularly as compared toconventional injection molding, the present unit suffices for the axialdisplacement of the screw member,

.despite the larger diameter of its outlet end 26. This arrangement hasthe result that the screw member is not overstressed by the torqueswhich occur during the mixing or plasticization, or by the axial forcesdeveloped during the injection step, and no special control measures arerequired. To adapt the plasticizing unit 11 and its associated screwmember 25 to different processing operations, it has been foundadvantageous to make the outlet end 26 and the head 27 screw memberremovable and exchangeable from the rest of the screw member body.Accordingly, as shown in FIG. 2, a outlet end 26' of the screw memberalong with the associated return flow block 28 which provides an openflow path 30 into the collecting chamber, can be replaceably connectedto the remainder of the screw member by a threaded member 60 which issecured within the core of the threaded screw member 25'. With thisarrangement, the entire unit can be adapted to different foaming plasticmaterial processing operations. The foregoing description of theappparatus and methods for its use refer to the delivery of a mixture ofplastic material and expanding agent into the plasticizer unit where theexpanding agent adheres either as a liquid film on the plastic granulesor is mixed as a powder with the plastic granules, or is uniformlydisplaced within the plastic granules.

It is possible to supply the expanding agent separately from the plasticmaterial at a point along the plasticizing unit within which thethreaded screw member rotates. The expanding agent can be charged intothe unit either in solid, liquid or gaseous form. As shown in FIG.- 2,an advantageous location for supplying the expanding agent is at thejunction between the first and second wall sections at the point wherethe outlet end 26' of the screw member 25 diverges outwardly,at thisparticular location the plastic material is under pressure and anadditional mixing takes place.

' One or several nozzles or inlet channels 61 are distributed about thecircumference of the unit and ex tend inwardly from a feed pipe 62 forsupplying the expanding agent. In certain plastic materials or expandingagents,may the advantageous to bring the expanding agent under pressurethrough a bore in the screw member and through a distributing device forcharging the expanding agent into the plastic material contained in thegrooves between the threads on' the screw member. The distributingdevice can be arranged adjacent the inlet end of the unit or adjacentthe outlet end of the threaded portion of the screw member, depending onthe properties of the specific material or of the expanding agent, whichwould also be true for the use of inlet channels passing through thewalls of the plasticizer unit.

With the method and apparatus described above, it it is possible toproduce the requisite foamed objects without providing a special plasticmaterial mixture in the form of granules and containing an expandingagent. Further, greater safety can be expected for the storage andhandling of the separate plastic material and expanding agent.

What is claimed is:

1. Apparatus for intermittent fabrication of objects of limited volume,having an imporous dense film surface enclosing a foam-like porous core,by injection molding of plastic material being accumulated in-a chamberand intermittently forced through a nozzle into a cavity, having alimited and wholly enclosed filling volume, of a split mold which can beopened in cadence with the intermittent fabrication of the objects, saidapparatus comprising in combination, an elongated plasticization andinjection cylinder extending from a material inlet to a materialinjection outlet communicating with said nozzle; and a plasticizationand injection screw mounted in said cylinder for rotation and axialdisplacement therein and extending therethrough; said cylinder includinga first elongated section, of substantially uniform diameter, extendingfrom said material inlet, and a second section extending from said firstsection'to said material injection outlet, and including an intermediateportion having a substantially uniform diameter larger than that of saidfirst section, a frusto-conical transition portion diverging from saidfirst section to said intermediate portion, and a further portionextending from said intermediate portion to said outlet; said furtherportion and at least a part of said intermediate portion constitutingsaid chamber; said screw having a first threaded section with an outerdiameter substantially equal to the inner diameter of said firstcylinder section and extending through said first cylinder section, anda second section in said second cylinder section including anintermediate portion having an outer diameter substantially equal to theinner diameter of said second cylinder section intermediate portion, atransition-threaded section having an outer surface conforming to theinner surface of said transition portion of said cylinder secondsection, and further portion extending from said screw intermediateportion into said further portion of said second cylinder section; theaxial length of said screw intermediate portion being less than theaxial length of the intermediate portion of said second cylindersection; said screw being axially displaceable rearwardly in saidcylinder, to obtain plasticization of the material, during accumulationof the material in said chamber and, after completion of plasticizationof the material, being axially displaceable forwardly in said cylinder,to inject plasticized material, accumulated in said chamber, into saidlimited volume mold cavity.

2. Apparatus for injection molding plastic objects, as set forth inclaim 1, wherein said screw intermediate portion has an annular groovetherein; and an annular return flow block member positioned in saidgroove for limited axial displacement relative thereto; said return flowblock member preventing return flow of the plastic material in thedirection of the threaded portion of said screw when the material isbeing extruded from the outlet of said cylinder by axial displacement ofsaid screw toward said outlet.

3. Apparatus for injection molding of plastic objects, as set forth inclaim I, wherein the threaded portion of said screw located within thefrusto-conical portion of said second cylinder section has threadsthereon for the extent of the axial dimension of the frusto-conicalportion of said second cylinder section.

4. Apparatus for injection molding of plastic objects, as set forth inclaim 2, wherein said return flow block member is spaced, in thedirection of the outlet opening, from the end of the threaded portion ofsaid screw within the frusto-conical portion of said second cylin ersection, and said block member has an outside diam ter corresponding tothe inside diameter of said intermediate portion of said second cylindersection.

5. Apparatus for injection molding of plastic objects, as set forth inclaim 1, wherein said second screw section is removable from andreplaceable on said first screw section.

6. Apparatus for injection molding plastic objects, as set forth inclaim 1, including a displacement member secured to said screw at itsend adjacent said material inlet for axially displacing said screw bothtoward and away from said material injection outlet; and means operableto move said displacement member axially through selected axialdistances in accordance with the parameters of the injection moldingoperation.

7. Apparatus for injection molding of plastic objects, as set forth inclaim 6, wherein means are arranged for rotating said screw about itslongitudinal axis and said displacement member is operative during theoperation of said rotating means to inhibit axial displacement of saidscrew away from said material injection outlet way.

8. Apparatus for injection molding of plastic objects, as set forth inclaim 1, wherein said first cylinder section has a first inlet openingspaced longitudinally from said second cylinder section for charging atleast plastic material into said cylinder.

9. Apparatus for injection molding of plastic objects, as set forth inclaim 8, wherein said first cylinder section has a second inlet openingspaced longitudinally from said first inlet opening toward said outletfor charging the expanding agent into said cylinder.

10. Apparatus for injection molding of plastic objects, as set forth inclaim 9, wherein the second inlet opening within said first cylindersection is positioned adjacent to the junction of said first and secondcylinder sections.

11. Apparatus for injection molding of plastic objects, as set forth inclaim 1, wherein said screw member comprises a core and a continuousthread projecting outwardly from said core, said core having a variablediameter for at least a portion of its length increasing in thedirection of the outlet, whereby the spaces defined between said coreand the opposed surfaces of said thread and the inside surface of saidcylinder vary along the axial direction of said screw.

12. Apparatus for injection molding plastic objects, as set forth inclaim 1, wherein said further portion of said screw is frusto-conicaland tapers toward said material injection outlet; said further portionof said second cylinder section being frusto-conical and convergingtoward said material injection outlet.

13. Apparatus for injection molding of plastic objects, as set forth inclaim 1, wherein at least one heater sleeve is located on each of saidfirst cylinder section and second cylinder section for supplying heat tothe materials con-veyed through said cylinder by said screw.

14. Apparatus for injection molding plastic objects, as set forth inclaim 1, wherein the axial length of said screw intermediate portion isless than the axial length of the intermediate portion of said secondcylinder section by an amount equal to the maximum axial displacement ofsaid screw in said cylinder.

1. Apparatus for intermittent fabrication of objects of limited volume,having an imporous dense film surface enclosing a foamlike porous core,by injection molding of plastic material being accumulated in a chamberand intermittently forced through a nozzle into a cavity, having alimited and wholly enclosed filling volume, of a split mold which can beopened in cadence with the intermittent fabrication of the objects, saidapparatus comprising in combination, an elongated plasticization andinjection cylinder extending from a material inlet to a materialinjection outlet communicating with said nozzle; and a plasticizationand injection screw mounted in said cylinder for rotation and axialdisplacement therein and extending therethrough; said cylinder includinga first elongated section, of substantially uniform diameter, extendingfrom said material inlet, and a second section extending from said firstsection to said material injection outlet, and including an intermediateportion having a substantially uniform diameter larger than that of saidfirst section, a frusto-conical transition portion diverging from saidfirst section to said intermediate portion, and a further portionextending from said intermediate portion to said outlet; said furtherportion and at least a part of said intermediate portion constitutingsaid chamber; said screw having a first threaded section with an outerdiameter substantially equal to the inner diameter of said firstcylinder section and extending through said first cylinder section, anda second section in said second cylinder section including anintermediate portion having an outer diameter substantially equal to theinner diameter of said second cylinder section intermediate portion, atransition-threaded section having an outer surface conforming to theinner surface of said transition portion of said cylinder secondsection, and further portion extending from said screw intermediateportion into said further portion of said second cylinder section; theaxial length of said screw intermediate portion being less than theaxial length of the intermediate portion of said second cylindersection; said screw being axially displaceable rearwardly in saidcylinder, to obtain plasticization of the material, during accumulationof the material in said chamber and, after completion of plasticizationof the material, being axially displaceable forwardly in said cylinder,to inject plasticized material, accumulated in said chamber, into saidlimited volume mold cavity.
 2. Apparatus for injection molding plasticobjects, as set forth in claim 1, wherein said screw intermediateportion has an annular groove therein; and an annular return flow blockmember positioned in said groove for limited axial displacement relativethereto; said return flow block member preventing return flow of theplastic material in the direction of the threaded portion of said screwwhen the material is being extruded from the outlet of said cylinder byaxial displacement of said screw toward said outlet.
 3. Apparatus forinjection molding of plastic objects, as set forth in claim 1, whereinthe threaded portion of said screw located within the frusto-conicalportion of said second cylinder section has threads thereon for theextent of the axial dimension of the frusto-conical portion of saidsecond cylinder section.
 4. Apparatus for injection molding of plasticobjects, as set forth in claim 2, wherein said return flow block memberis spaced, in the direction of the outlet opening, from the end of thethreaded portion of said screw within the frusto-conical portion of saidsecond cylinder section, and said block member has an outside diametercorresponding to the inside diameter of said intermediate portion ofsaid second cylinder section.
 5. Apparatus for injection molding ofplastic objects, as set forth in claim 1, wherein said second screwsection is removable from and replaceable on said first screw section.6. Apparatus for injection molding plastic objects, as set forth inclaim 1, including a displacement member secured to said screw at itsend adjacent said material inlet for axially displacing said screw bothtoward and away from said material injection outlet; and means operableto move said displacement member axially through selected axialdistances in accordance with the parameters of the injection moldingoperation.
 7. Apparatus for injection molding of plastic objects, as setforth in claim 6, wherein means are arranged for rotating said screwabout its longitudinal axis and said displacement member is operativeduring the operation of said rotating means to inhibit axialdisplacement of said screw away from said material injection outlet way.8. Apparatus for injection molding of plastic objects, as set forth inclaim 1, wherein said first cylinder section has a first inlet openingspaced longitudinally from said second cylinder section for charging atleast plastic material into said cylinder.
 9. Apparatus for injectionmolding of plastic objects, as set forth in claim 8, wherein said firstcylinder section has a second inlet opening spaced longitudinally fromsaid first inlet opening toward said outlet for charging the expandingagent into said cylinder.
 10. Apparatus for injection molding of plasticobjects, as set forth in claim 9, wherein the second inlet openingwithin said firsT cylinder section is positioned adjacent to thejunction of said first and second cylinder sections.
 11. Apparatus forinjection molding of plastic objects, as set forth in claim 1, whereinsaid screw member comprises a core and a continuous thread projectingoutwardly from said core, said core having a variable diameter for atleast a portion of its length increasing in the direction of the outlet,whereby the spaces defined between said core and the opposed surfaces ofsaid thread and the inside surface of said cylinder vary along the axialdirection of said screw.
 12. Apparatus for injection molding plasticobjects, as set forth in claim 1, wherein said further portion of saidscrew is frusto-conical and tapers toward said material injectionoutlet; said further portion of said second cylinder section beingfrusto-conical and converging toward said material injection outlet. 13.Apparatus for injection molding of plastic objects, as set forth inclaim 1, wherein at least one heater sleeve is located on each of saidfirst cylinder section and second cylinder section for supplying heat tothe materials con-veyed through said cylinder by said screw. 14.Apparatus for injection molding plastic objects, as set forth in claim1, wherein the axial length of said screw intermediate portion is lessthan the axial length of the intermediate portion of said secondcylinder section by an amount equal to the maximum axial displacement ofsaid screw in said cylinder.